Differential circuit implementations are widely used for realizing mixed-signal systems owing to their ability to substantially reject (as common-mode) ambient noise signals e.g. supply noise, reference noise and substrate noise that can ruin signal integrity in single-ended circuit implementations. An issue to address when designing fully differential amplifiers is designing a common-mode voltage control loop to set the differential output common-mode voltage.
Common-mode control requires methods for both sensing and forcing the differential output voltage's common-mode voltage. Typical requirements for common-mode sensing are to achieve sense circuit linearity to avoid common-mode to differential signal conversion.
Disadvantages with prior art include high sensitivity to any offset or device matching errors, causing large common-mode dc voltage errors, and also reduced bandwidth leading to extended settling time of the common-mode control loop in response to common-mode disturbances.
It is a challenge to designers of fully differential amplifiers to overcome these disadvantages.